Method of producing plastic tubing having a corrugated outer wall

ABSTRACT

Apparatus for producing a plastic pipe or tubing having concentric juxtaposed bonded internal smooth wall tubing and external transversely corrugated wall tubing which may additionally have a further concentric more external smooth wall tubing, which apparatus comprises extrusion means for extruding at least two separate streams of thermoplastic material; means for imparting transverse corrugations to the external of these two extruded streams; and means for juxtaposing and bonding the internally extruded smooth walled tubing to the innermost surfaces of the externally disposed transversely corrugated tubing. The molding means are recirculating bipartite molds which travel in the direction of extrusion of the tubing. The internal smooth-walled tubing is juxtaposed to external tubing after such has been molded into a transversely corrugated shape.

Nov. 3, 1970 w; HEGLER 3,538,209

METHOD OF PRODUCING PLASTIC TUBING HAVING A CORRUGATED OUTER WALL FiledOct. .19, 1967 5 Sheets-Sheet 1 FIG. 1.

'' il IIIIIEI INVENTOR WILHELM 'HEGLER WMS W AT TORNEYS.

Nov. 3, 1970 w. HEGLER METHOD OF PRODUCING PLASTIC TUBINGHAVING ACORRUGATED OUTER WALL Filed Oct. 19, 1967 3 Sheets-Sheet 2 IINVENTORWILHELM" HEGLER BY M, M9 W AT TORNEYS.

INVENTOR AT TORNEYS.

NOV. 3, 1970 HEGLER METHOD OF PRODUCING PLASTIC TUBING HAVING ACORRUGATED OUTER WALL 3 Sheets-Sheet 5 Filed Oct. 19, 1967 WILHELMHEGLER W Mq" United States Patent 3,533,209 METHOD OF PRQDUCING PLASTICTUBING HAVING A CORRUGATED OUTER WALL Wilhelm Hegler, Gocthestrasse 2,Bad Kissingen, Germany Filed Oct. 19, 1967, Ser. No. 676,430 Claimspriority, application Austria, Feb. 27, 1967, A 1,880/67 Int. Cl. B29c17/07, 17/10; 132% 23/03 US. Cl. 26490 8 Claims ABSTRACT OF THEDISCLOSURE A method for producing a corrugated, double-walled plasticpipe or tube by extrusion and vacuum forming processes. Two separatetubular streams are simultaneously extruded from separate extruders andare concentrically disposed. The innermost tubular stream is maintainedin a substantially smooth wall condition while the outermost tubularstream is disposed against longitudinally moving concentric transverserib forming molds and formed against the molds with suction. Theinnermost tubular stream is then adhered to the inside surfaces of theformed corrugations of the outermost tubular stream. The extrusion ratesof the streams are adjusted to maintain a constant thickness in theformed and adhered tubes. A third tubular stream having a smooth wallconfiguration may be extruded and adhered to the outer surface of thecorrugated tube if desired.

Smooth plastic pipes that are extruded from annular dies have thedisadvantage that they have poor diametral compressive strength, thatis, they are easy to collapse by squeezing them. In order to give themthe necessary strength, they must have considerable wall thickness.Smooth plastic pipe, even of the thin variety having less thansatisfactory flexibility, has even less flexibility where the pipe wallthickness is increased in order to impart greater diametral strengththereto.

Attempts have been made to impart greater diametral strength to plasticpipe without overly decreasing the flexibility thereof by providingsolid annular or helical ribs therearound. While improvements indiametral strength without extreme reduction in flexibility has beenaccomplished by this technique, it would still be desirable to furtherimprove this situation.

Corrugated pipes can be made by a process in which a tube extruded froman annular die is carried in the still plastic state between continuallyrecirculating bipartite sets of hollow molds which are provided withcorrugations, and the tube is drawn by an external vacuum or forced byinternal pressure into the corrugations in the hollow molds. Thesetubes, however, again have the disadvantage that their inside surface isnot smooth, but is likewise corrugated, so that such pipes cannot beused for purposes in which a smooth inside surface is required, e.g.,for sewerage service or air conditioning or ventilating ducts and pipethrough which liquid is to be pumped. Such pipes are not suitable aspressure piping, either, since a change in pressure inside of the pipeproduces a change in its length. Furthermore, pipes made of plastic areknown, especially drain pipes, which have annular or helical reinforcingribs on their outside surface and a smooth inside surface. Such pipeshave a higher diametral compressive strength than smooth extrudedplastic pipe, but they have very poor flexibility, since the alreadypoor flexibility of the plain pipe is even further impaired by the solidribs created on it.

It is also in the prior art to extrude two plastic pipes 3,538,209Patented Nov. 3, 1970 concentrically to one another, using a dualannular extruding die, longitudinal ribs being able to be formed on theinside pipe to space it off from the outside pipe. These pipes, however,are still less flexible than the single, plain plastic pipes.

It is therefore an object of this invention to provide a novel method ofmanufacturing a plastic pipe having high diametral strength as comparedwith pipe of the prior art.

It is another object of this invention to provide a novel method ofmanufacturing such improved diametral strength plastic pipe withflexibility which is not significantly reduced as compared with priorart pipe.

Other and additional objects of this invention will become apparent froma consideration of this entire specification including the drawing andclaims hereof.

In accord with and fulfilling these objects, one aspect of thisinvention resides in a plastic pipe having a smooth inside wall and aribbed wall external thereof wherein the ribs are hollow and thus formcorrugations. It is within the scope of this aspect of this invention toprovide a pipe having a smooth inner wall and a corrugated outer wall.It is further within the scope of this aspect of this invention toprovide a pipe having smooth inner and outer walls and corrugationtherebetweeen.

Another aspect of this invention resides in the process of producingreinforced plastic pipe as generally described above. According to thisaspect of this invention, plastic pipe is formed by multiwall extrusiontechniques. Where a pipe is desired .having a smooth inner wall and acorrugated outer wall, such is produced by extrusion through twoconcentric dies such that the outer wall is formed first and, whilestill in the plastic state, is corrugated through the use of internalfluid pressure, external vacuum or a combination thereof forcing theformed tubular wall into annular or helical molds in a serpentinemanner. The smooth inner wall is extruded as a tube within thisserpentine walled outer tube with an outside diameter substantiallyequal to the smallest inside diameter of the serpentine folds. It ispreferred to so extrude the smooth inner tube over an extended mandrelwhich causes such inner tube to bear against and adhere to the smallestinside diameter portions of the serpentine folded outer tube and thusclose off each serpentine fold and so form hollow ribs or corrugations.

Where a pipe is desired having a smooth outer wall, it is within thescope of this invention to provide a third concentric extruder such asto form an outer smooth tube having an inside diameter substantiallyequal to the largest outside diameter of the referred to serpentinefolds and bearing thereagainst in order to adhere thereto. It is ofcourse within the spirit and scope of this invention to provideadditional corrugated walls and/or additional smooth walls inalternating relation as desired for pipe of any particular total wallthickness desired, rigidity and diametral strength.

As used herein the term concentric extrusion is intended to mean thateach successive wall is extruded concentric to its adjacent wall. Suchterm is in no way intended to be limited to an absolutely simultaneous,concentric wall formation. In fact, it is within the scope of thisinvention to extrude a next adjacent outer tube of either smooth orserpentine configuration concentric to an already preformed nextadjacent inner tube or tube composite. Further, in the case whereplastic pipe of finite length is produced according to this invention,it may even be practical, under some circumstances, to extrude a nextadjacent inner tube of either smooth or serpentine configurationconcentric to and within an already preformed next adjacent outer tube.

It will be appreciated that while for some purposes press-fit adjacenttubes will suflice, for most purposes it will be desirable that adjacenttubular portions of the pipe of this invention be joined together in amore adherent manner and with less stress upon the members (which stressmight result in cold flow of one or both members). Thus it is preferredthat at least one of two adjacent tube members be in an adherent stateupon production of this pipe. This adherent state may be caused by atube being still at least partially thermoplastic upon contact with itsnext adjacent member, or such may be provided through the use ofsuitable adhesives. The thermo plastic adherable state is to bepreferred since by this means no external material is or need be addedto the system.

Where two or more walls of the multi-wall pipe of this invention are tobe produced substantially simultaneously, a common feed of plasticmaterial to a single extruder with two dies can be used. Alternately,separate feeds can be provided to each die of a single extruder. Thus itwill be apparent that the individual wall members of the pipe of thisinvention can consist of the same or different plastic materials.

Understanding of this invention will be facilitated by reference to theaccompanying drawing in which:

FIG. 1 shows an apparatus for the practice of the method of theinvention in a longitudinal section in which both pipes are manufacturedfrom the same material by the use of two different extruding machines.

FIG. 2 shows another apparatus for the manufacture of larger sizes ofpipe.

FIG. 3 shows a pipe manufactured by the process of the invention,partially in elevation, partially with the outer covering cut away, andpartially in section.

FIG. 4 shows in longitudinal and cross section, respectively, a drainpipe manufactured by the method of the invention and having orifices forthe admission of water.

According to the invention, two concentric pipes can be extruded in asingle operation, the outer pipe being guided in a prior art manner,after emerging from the annular extruding die, between continuallyrecirculating bipartite sets of hollow molds and being sucked by avacuum into the corrugations of the molds, while the inside pipe isextruded from a die located within the train of assembled hollow molds,the latter die being so arranged that the smooth inside pipe appliesitself to the completely formed outer corrugated pipe. In this manner apipe having a smooth inside wall is produced, in which the externalmechanical stresses are taken up by the corrugated outer wall, so thatthe pipe has a high compressive diametral strength, while the insidepipe can be relatively thin-walled so that such pipes are still quiteflexible.

The extruding of the pipes can be accomplished in a known manner byusing a single extruding machine which supplies the molding compound forboth pipes, that is, which feeds both of the annular extruding dies. Theouter wall and inner wall in this case consist of the same material.However, each of the two annular dies can be fed from its own separateextruding machine. This makes it possible for the outer wall and innerwall to be made of different materials, e.g., the outer wall of arelatively hard material which provides the necessary diametralcompressive strength, while the inner wall can consist of a soft andelastic material, thereby assuring a higher pipe flexibility. I

In the practice of the method of the invention, the outer pipe canemerge from the annular die at a speed which is greater than the speedwith which the hollow molds are carried on their path, while the innerpipe can emerge from the annular die at a speed which is equal to orlower than the speed at which the hollow molds are carried on theirpath. In this manner the assurance can be had that the inside pipeapplies itself to the outside pipe at all points at which the outsidepipe has corrugations; furthermore, both the material of the outsidewall and the material of the inside wall are to be still suflicientlyplastic at the point at which this contact takes place that a permanentbond is created between the two pipes. On the other hand, however, theinside pipe can be kept under a certain tension so as not to sink intothe corrugations of the outside pipe before hardening, which wouldproduce a corrugation on the inside pipe, too.

If the pipes are to be used as drain pipes or irrigating pipes or forother such purposes, in which inlet or outlet orifices must be providedin the wall, longitudinal channels can be produced in the inside wallwhich cross the transverse corrugations in the outside wall, while thehollow molds have at their projecting portions matching recesses, sothat protuberances are produced in the valleys between corrugations andthe longitudinal channels running through these protuberances. Theseprotuberances can then be cut open to produce the orifices.

In this case, it is best to make the external transverse corrugationshelical in form, so that the tool for cutting open the protuberances canbe guided in the helix. At the same time the wall of the pipe can alsobe designed in the form of a double or multiple helix and, depending onthe number of orifices necessary, orifices can be located in only onehelix of a double-helical pipe, for example.

If a pipe manufactured by the process of the invention must also besmooth on the outside, a third, smooth external pipe can be extrudedover the pipe in a further manufacturing operation. This outside pipecan again be made of a different material than the inner pipe, a softermaterial, for instance, so as to impair the flexibility of the entirepipe as little as possible.

A further aspect of this invention is an apparatus for the practice ofthe method of the invention. An extruding apparatus must be used whichhas two annular dies, the die that produces the inner pipe beingdisposed expediently in a prolongation of the mandrel of the annular diethat produces the outside pipe, this prolongation being made so longthat the annular die for producing the inside pipe is located within thetrain of closed hollow molds and the annular die for producing theoutside pipe is located in the first part of or outside of the train ofclosed hollow molds. In order that the inside pipe may be applied firmlyto the outside pipe and be welded to the latter, a mandrel prolongationcan be disposed on the annular die that produces the inside pipe, thediameter of the said mandrel being at least approximately the same asthe inside diameter of the outside, corrugated pipe, so that thismandrel prolongation will urge the inside pipe against the outside pipewhich is inside of the hollow molds and supported by them. If a pipe isto be made which can be used as a drain pipe with orifices for lettingWater in or out, the longitudinal channels can be produced in the insidewall of the inside pipe by corresponding projections on this mandrelprolongation, which force the inside pipe against the outside pipe alsoat the points at which the projecting parts of the hollow molds haverecesses.

If the metal tube bearing the annular die for the production of theoutside pipe extends into the train of closed hollow molds, the distancebetween the annular dies and the extruding machine becomes relativelygreat. As a result of the sucking out of the air from the grooves in thehollow molds, air is constantly flowing in from the exterior along thistube and cools it. In order to prevent the plastic in this tube fromcooling below the softening temperature, the tube should be equippedwith a heating band or a thermally insulating covering, especially onemade of asbestos.

In the apparatus of FIG. 1 the plastic coming from the extremity 22 of afirst extruding machine is fed through a mounting ring 23 to a die 5while the plastic coming from a second extruding machine is fed througha fitting 24 and an additional hole in the mounting ring 23 to a die 8.The plastic coming from this second extruding machine constitutes theinterior stream of plastic 4. The outside stream emerges from an annulardie 5 as a smooth tube,

a mandrel 6 of the said die being provided with a prolongation 7 bears asecond die 8, whose mandrel 9 is held by a second mounting ring 10. Itis desirable for the die 8 and the mandrel 9 to be so shaped that theinner pipe, as represented, is extruded at a sharp angle against theoutside pipe which in the meantime has been corrugated. There isprovided on the mandrel 9 a mandrel prolongation 11 whose outsidediameter is the same as the desired inside diameter of pipe to be made,i.e., the diameter is smaller than the minimum inside diameter of thehollow molds by the thickness of the tube being manufactured.

The smooth pipe emerging from the die passes between hollow bipartitemolds 12 which are circulating in the manner of crawler treads. Air ispumped out of the grooves 13 in the hollow molds, by means of a vacuumline 15, as soon as the molds are fully closed. As is shown in thedrawing, the vacuum draws the previously smooth outer tube into thegrooves in the molds thereby drawing the tube against the molds. Sincein order for this to be possible normal air pressure must prevail insideof the outside pipe, holes 16 are provided in the mounting ring 2 andholes 17 are provided in mandrel 6, through which, as indicated byarrows 18 and a broken line 19, the air is able to enter. Throughadditional holes 20, in mandrel 9, and 21 in the mounting ring, air canalso flow from the inside pipe into the interior of the outside pipe.The guidance of hollow bipartite molds 12 is not shown in detail, sincebipartite molds of this type which are circulated in the manner ofcrawler treads are known to the prior art. After the outside pipe andinside pipe have passed the mandrel prolongation 11 and have been joinedtogether, the hollow molds are cooled below the hardening temperature ofthe plastic, and the finished pipe continuously emerges from the openingmolds.

In the apparatus of FIG. 2, the extruding machines smaller diameters.The die 26 and the corresponding mandrel 27 are in this case so designedthat the emerging plastic is extruded at an acute angle into grooves 28in molds 29, thus aiding the formation of transverse corrugations. Theaspiration of the air from grooves 28 by means of a vacuum line 30 inthis case can best begin at the point at which the plastic is injectedinto the grooves in order to form the outer pipe. As in the apparatus ofFIG. 1, here again the mandrel 33 and the mandrel prolongation 34, bywhich the inside pipe is forced against the inner folds of the outsidetube and is thereby welded to same, are disposed on the mounting ring 31of a die 32.

As already mentioned, the result of asplratlon of air from the grooves28 is that air flows in from the left through the closing molds 29 alongpipe 35. This could result in an excessive cooling off of pipe 35, whichcould result in trouble and in the danger of explosion. For this reason,pipe is surrounded by a heating band or by a thermally insulatingcovering 36. This thermal insulation is held in place by a clampingmeans 37.

The pipe manufactured with one of the apparatus of FIGS. 1 or 2 can, ifdesired, have a third smooth outer tube extruded around it afterwards.Such a pipe is shown in FIG. 3 partially in section. This pipe consistsof the smooth inside pipe 40, the transversely ribbed outer pipe 41, andthe smooth outer pipe 42 drawn over the latter. Tubes 40 and 42 canconsist of a relatively soft or elastic plastic, and can have arelatively thin wall, because neither of these tubes have to withstandmechanical stresses, though they must permit themselves to be elongatedor compressed when the entire pipe is bent. Tube 41, however, isrelatively thick-walled or consists of a relatively hard plastic, sothat it has the required diametral compressive strength, and this tubeis designed to take the mechanical stresses. FIG. 3 shows, in harmonywith FIG. 2, that in a pipe manufactured by the method of the inventionthe helical ribs do not consist of solid material but are hollow.

FIG. 4 shows in longitudinal and cross section a pipe manufactured bythe method of the invention and having orifices for the admission orexhaustion of water. In the manufacture of this pipe, the bipartitehollow molds have recesses on their inwardly projecting portions, sothat the protuberances 45 are created in the exterior valleys betweenthe corrugations. At the corresponding points, the mandrel prolongation7 in FIG. 2, and prolongation 34 in FIG. 2, have projections which forcethe wall of the inside and outside pipe outwards, so that thelongitudinal grooves 46 develop on the inside. A cutting tool is thenguided about the finished pipe leaving the train of hollow molds andcuts off the protuberances 45, i.e., removes the portion that is notshown crosshatched in FIG. 4, so that the orifices 47 are formed at thepoints at which the outer transverse grooves 48 intersect the innerlongitudinal grooves 46. FIG. 4 it is assumed that the outer transversegrooves form a double helix, only the protuberances in every othergroove being cut off. If a larger number of water admission orifices isdesired, or a larger total orifice cross section, the protuberances inthe transverse grooves between them can also be cut off, in which casecutting tools are guided around the pipe in both helixes.

In accordance with the practice of this invention, the pipe can consistof substantially any plastic material which is suitable for extrusion,particularly extrusion to produce pipe. Exemplary of such materials arevinyl chloride homoand copolymers, acrylonitrile homoand copolymers,olefin polymers, vinyl ester and ether polymers, styrene polymers,polycarbonates, polyepoxides, polyurethanes, polyphenylene oxides,condensation polymers of the amide or ester type, etc.

What is claimed is:

1. Process of producing plastic pipe having a smooth inner wall andsubstantially hollow transverse ribs therearound, which inner wall isbonded to the smallest inside diameter portions of said transverse ribs,which process comprises substantially simultaneously extruding at leasttwo separate, substantially concentric, tubular streams of said plasticfrom a corresponding number of extruders; maintaining the innermost ofsaid tubular streams in a substantially constant inside and outsidediameter smooth wall condition; disposing the next to the innermost ofsaid tubular streams against longitudinally moving outwardly concentrictransverse rib form mold means; applying a vacuum through said moldmeans whereby inducing said next inner-most tubular stream to conform tosaid mold means while maintaining a substantially uniform wall thicknesstherein, where the innermost diameter of said rib-molded tubesubstantially corresponds to the outermost diameter of said innermostsmooth walled tube; and adhering said innermost and outermost diameterstogether whereby forming a smooth innerwalled tube having transversehollow ribs disposed therearound, wherein the innermost and nextinnermost tubular streams are extruded at different flow ratessufiicient to form said smooth innermost tube and said next innermostrib molded tube at substantially the same longitudinal linear speed.

2. Process as claimed in claim 1 wherein said next innermost stream isextruded as a substnatially smooth first wall tube member, is passedthrough longitudinally moving bipartite hollow serpentine mold meanswhile still in the plastic state, is formed into said ribbed tube bysaid molds during movement thereof, and is adheredto said smooth innertube wall member after molding thereof into said transverse tubeconfiguration.

3. Process as claimed in claim 2 including extruding said next innermoststream at a linear flow rate sufiiciently higher than the linear rate ofany point on said bipartite mold means to maintain the wall thickness ofsaid next innermost tube substantially the same prior to, during andafter molding thereof to the shape of said bipartite mold means; andextruding said innermost stream at such a linear flow rate that the wallthickness thereof is maintained substantially constant before, duringand after adherence of said innermost tube to said next innermost tube.

4. Process as claimed in claim 2 wherein said transverse ribbed tubewall member emerges from said molds at a speed at least as high as thespeed of extrusion of said inner smooth tube wall member.

5. Process as claimed in claim 2, wherein said substantially smoothfirst tube member is extruded at a speed greater than the speed at whichsaid ribbed tube wall member emerges from said bipartite hollowserpentine molds.

6. Process as claimed in claim 1 carried out continu ously.

7. Process as claimed in claim 1 including substantially simultaneouslyforming a second smooth tube wall mem ber, having an inside diametersubstantially equal to the largest outside diameter of said nextinnermost tube wall member, concentric with and outside said nextinnermost tube wall member and in intimate contact therewith along atleast some of the largest outside diameter surfaces thereof.

8. Process as claimed in claim 1 including forming protuberances on theouter side of the smaller diameter portions of said ribbed tube Wallmember; forming longitudinal grooves on said inner smooth tube wallmember in registry with said protuberances; and then cutting theseprotuberances so as to form apertures in said pipe.

References Cited UNITED STATES PATENTS ROBERT F. WHITE, Primary ExaminerJ. H. SILBAUGH, Assistant Examiner US. Cl. X-R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,538,209 Dated November 3, 1970 Inventor(s) Wilhelm Hegler It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 3, line 33, "4 shows" should read 4 and 5 show Column 6, line 8"Fig 4" should read Figs 4 and 5 line 15 "Fig. 2" should read Fig l line22 "Fig 4" should read Fig. 5

Signed and sealed this 25th day of May 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, J1 Attesting OfficerCommissioner of Patent:

\ FORM PO-105O (lo-s9) USCOMWDC

